feat: initial m3ta-home structure

- flake.nix with nixpkgs, home-manager, nix-colors, m3ta-nixpkgs, agenix, NUR inputs
- lib/mkHome.nix: compose HM config from user + identity + context + sets
- profiles/base: shell, cli-tools, secrets (always loaded)
- profiles/contexts/desktop: WM, apps, theme, ghostty
- profiles/contexts/server: minimal headless
- profiles/sets/coding: core (git, direnv, jq, rg), editor, lsp, languages, agents
- profiles/sets/gaming: steam, gamescope, gpu
- profiles/sets/media: obs, ffmpeg, kdenlive, handbrake, yt-dlp
- users/m3tam3re/identities: private.nix, work.nix (git, jj, ssh per identity)
- users/m3tam3re/preferences: cliphist, difftastic, base packages

profiles/sets/gaming/steam.nix

@@ -0,0 +1,21 @@
+# Steam gaming platform with Steam Deck compatibility tools and gaming utilities.
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+with lib; let
+  cfg = config.profiles.gaming.steam;
+in {
+  options.profiles.gaming.steam.enable = mkEnableOption "enable Steam gaming";
+
+  config = mkIf cfg.enable {
+    home.packages = with pkgs; [
+      gamemode
+      goverlay
+      mangohud
+      protonplus
+    ];
+  };
+}